A. Field of the Invention
The device of the present invention generally relates to nuclear fuel assemblies for use in nuclear reactors, and, more particularly, to a new and improved nuclear fuel assembly especially adapted for use in a boiling water reactor.
B. Description of the Prior Art
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is old and now well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of the nuclear fuel rods are grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical nuclear fuel assembly may be formed by a 7.times.7 or an 8.times.8 array of spaced-apart, elongated rods supported between upper and lower tie plates. Examples of such typical nuclear fuel assemblies are depicted in and described in U.S. Pat. Nos. 3,350,275; 3,466,226 and 3,802,995. In a typical boiling water reactor nuclear fuel assembly having an 8.times.8 rod array, the sixty-four rods that form the 8.times.8 array may be either sixty-four fuel rods or one or more non-fueled, water moderator rods with the remaining rods being fuel rods. A common problem in typical boiling water reactor nuclear fuel assemblies of the types depicted in the above-identified patents is that the central region of such fuel assemblies may be under-moderated and over enriched. In order to increase the flow of moderator, one or more elongated, water moderator rods have been substituted for fuel rods in the central region of such nuclear fuel assemblies. For example, water moderator rods 41 and 42 are depicted in and described in the above-identified U.S. Pat. No. 3,802,955. The use of one or more water moderator rods, however, prevents the use of a full complement of 64 fuel rods, i.e., one fuel rod in each rod lattice position within the fuel assembly. Thus, a higher linear heat generation rate may be required when one or more water moderator rods are used in place of one or more fuel rods within the fuel assembly.
In the above mentioned typical boiling water reactor nuclear fuel assembly, the outer flow channel has been formed of an alloy of zirconium known as Zircaloy and is structurally unsupported across its cross section perpendicular to its longitudinal axis. At reactor operating conditions, such an outer flow channel deforms slightly due to creep. Currently, attempts are being made to counteract the creep deformation by using an outer flow channel with thicker walls in order to extend its life. However, using thicker walls increases parasitic absorption in the reactor core with the resultant increase in fuel cycle costs.
Many nuclear fuel assemblies now being used in boiling water reactors have lifting handles bolted to their upper tie plates for handling and moving the fuel assemblies in a vertical position. Thus, the weight of each fuel assembly, including the weight of its outer flow channel, is transmitted from the lifting handle through a plurality of fuel rods commonly referred to as tie rods equipped with special end plug fittings that pass through the upper and lower tie plates of the fuel assembly and are captured with locking nuts. Such tie rods are stressed during handling maneuvers; and such stresses may give rise to fuel rod failures.